This is some good info on leaving a CNC machine unattended and yet it still will not prevent this from happening. I am humbled by the fact that I have caught my machine on fire and keep fire extinguishers near at all times. As with any tool there are inherent dangers. I have come a bit further with the use of my machine and used to cut too far into the spoil board (now corrected) yet this was caused by running someone elseís file and not pre-checking the code.
From contributor T:
I am impressed with your openness, candor, and willingness to share the "other side" of what happens when things go wrong. If you do not want to replace the table, bondo or epoxy the damaged area and re-machine to specs. I have done this dozens of times and it works well.
I smelled the burning first, looked in and saw the scorching and the sparks fly on every entry and shut off the machine. Close call, but we would never have cut that unattended, only I do that after regular hours. I was amazed the router cut the galvanized at all, and actually pretty good for a little while, but lesson learned.
Another story, while on the topic of fires, was that one of my competitors was presumably running a job at night and had the air compressor on (for tool changes or cooling I would think). It blew a fitting, started running non-stop and eventually caught on fire, possibly due to dust around or on top of it. His insurance barely covered the initial clean up! After that, I now worry as much about leaving the compressor power on as I do the CNC running.
As far as checking the code you never leave code running you aren't really sure of. One of my first CNC mistakes was not checking/understanding the code. I scaled a set of letters 12x, not realizing that I also scaled the depth to 12" deep from 1" deep. I use mainly 1/4" bits now, but was using a 3/8" - 1/2" behemoth at the time, about 6" long (for foam). It went right past the spoilboard into the aluminum extrusion below and then started trying to cut the letters! I had a "trophy case" of aluminum bits in foam for a while after than one.